In the field of matrix printers, it is of course well known that a printer may include one or more print heads which are caused to be moved in a reciprocating manner across the printer for printing in a serial manner. The print head may be moved by a cable and pulley arrangement, a lead screw, or a cam drive or like drive mechanism. Each of the print heads includes a plurality of elements supported in a group and actuated or energized at high speed to cause printing of dots by the movement of dot-making elements, including droplets of ink or print wires attached to solenoids which are caused to be impacted against the paper. The print wires or ink jet nozzles are usually spaced vertically so as to print the dots making up the characters in a line as the print head is moved across the printer. In this manner, a line of printed characters is completed upon travel of the print head in one direction across the paper.
Another form of matrix printer includes the use of a plurality of printing elements supported from a carriage in a manner wherein the elements are aligned horizontally across the printer and upon each pass of the carriage respective dots of characters are printed in a line or row and subsequent passes of the carriage and printing elements cause additional lines of dots to be printed to complete the dot matrix characters along the line of printing. Common arrangements include the use of four or eight printing elements supported from the carriage.
A timing strip with slots or like indicia is commonly used to originate the actuation of the printing elements wherein one or more sensors sense the slots or other indicia to print dots in precise columns across the paper. While the printing has usually been performed in one direction, for example, left to right, more recently the printing has been done in both directions of travel of the print head carriage or of the printing element carriage.
The speed of the motor driving the print head influences the location of each print dot that forms a matrix of dots creating a character. The motor's speed is in turn influenced by the amount of torque applied to the motor's input shaft, the temperature, the humidity, mechanical wear and other miscellaneous factors. Motor speed controllers are used to attempt to maintain the speed of the motor at a relative constant to insure quality printing, but it has been determined that speed controllers generally can only control speed within a limited range and when the loading on the motor exceeds this range, the motor's speed is not accurately controlled.
It therefore is desirable to have a motor speed controller which can accommodate a shift in the loading range of a motor so as to maintain the motor's speed substantially constant.
A patent of interest for its showing of a printer of the type wherein the present invention finds particular utility is U.S. Pat. No. 4,204,777 entitled "Matrix Printer Control System", by T-K Jen the inventor of the present invention.
The printer of this patent is of the type wherein printing is performed in each direction of travel of the print head and each dot of the printed characters must be deposited or positioned at the same column location for that particular dot in each direction of printing. The quality of the motor speed controller for this type of printer will bear a direct relationship upon the alignment of the dots in the printed characters.
Another patent of interest is U.S. Pat. No. 4,208,137 entitled "Position Sensing for Matrix Printer", by C. H. Liu. In this patent, the aforementioned printer of T-K Jen is improved with, among other things, a back electromotive force direct current motor speed control circuit.
Another motor speed controller of interest is disclosed in U.S. Pat. No. 4,280,082 entitled "Digital DC Motor Speed Control Circuit", by R. M. Acharya et al. This patent is assigned to NCR Corporation as are the two preceding referenced patents and the present application. The speed control circuit of this patent maintains the speed of a high current DC motor at a constant speed over a varying supply voltage and varying torque loads. The speed of the motor is controlled by adjusting the width of motor voltage pulses as a function of the speed of the motor and the level of the motor supply voltage such that the voltage pulses have a proportionally larger width for a smaller level of motor supply voltage or for a decrease in the amount of motor shaft speed and a proportionally narrower width for increased levels of motor supply voltage or for an increase in the amount of motor shaft speed.